wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

2-16 Industrial Communication Systems
needs to be equalized by the receiver that requires estimation of the channel. If the channel
response has multiple zeros in the frequency domain, it might happen that equalization is not
possible any longer, resulting in a high BER.
The OFDM approach is different to that: the single, high bit rate carrier is split into multiple
lower bit rate carriers (multicarrier modulation). This has some benefits: the sub-channels have
low bandwidth and flat fading can be assumed for each sub-channel (even if the entire bandwidth
across all sub-channels shows frequency selective fading). Hence, now channel estimation/
equalization is necessary. Furthermore, if the channel’s frequency response has nulls on some
sub-carriers, only those show bit errors. By appropriate coding, the overall BER can be kept low.
• Ultra wideband (UWB): This very new technology spreads the data bits over bandwidths in excess
of 500.MHz. This can be done by the generation of extremely short pulses in the range of some
100.ps for example. This results in high immunity to multipath fading while having the potential
to provide very high data rates (>500.MBit/s). UWB reuses frequency bands already occupied by
other communication systems. Due to UWB’s broadband nature, the spectral power density is
very low; existing communication systems are hardly affected by UWB.
2.4.4 Bit Coding, Multiplexing
2.4.4.1 Bit Coding
Because of the relatively high BERs of a wireless channel (due to fading), much effort has to be spent on
forward error correction. Typically, two different types of bit errors need to be considered:
• Single bit errors: Due to very small received signal strength the SNR decreases, leading to higher
BERs. By using error correcting codes, it is possible to dramatically reduce the BER by sacrificing
the data rate. The IEEE 802.11x standard uses coding rates down to 0.5, meaning each transmitted
bit is accompanied by an error-correction bit and, thus, reducing the usable bit rate by 50%.
• Burst errors: Because of the time-varying fading properties of a wireless link, a complete loss of
the reception often occurs for short time intervals. Because error-correction codes cannot deal
with burst errors, interleaving will be applied. This means that the data bits are spread over
time: If a short burst error occurs, the burst error is spread over the bit stream by the receiver’s
de-interleaver. As a consequence, the block of erroneous bits is spread into multiple single bit
errors. Now, the error-correction code can be used for calculating the correct bits.
2.4.4.2 Multiplexing
Sharing a wireless frequency band can be done by one or a combination of the following three ways:
• Frequency division multiple access (FDMA): Each communication link uses its own frequency.
Hence, no coordination between the different links is required.
• Time division multiple access (TDMA): Multiple users share one frequency on the basis of time
slots. This access scheme requires coordination of all transmitters.
• Code domain multiple access (CDMA): Here, all users use the same frequency at the same time
(continuously). To distinguish between multiple users, each transmitter encodes the data bits
by a spreading sequence (direct sequence spread spectrum, DSSS). By using different spreading
sequences with low cross-correlation, it is possible to separately extract a single transmitter’s bits
at the receiver side for each user.
• Frequency hopping spread spectrum (FHSS): By rapidly changing the center frequency in a pseudorandom
sequence (known by the transmitter and receiver), the transmission is spread in bandwidth.
This is not a real multiplexing technique as transmissions on the same frequency can occur
but helps to reduce BER in multiuser applications.
© 2011 by Taylor and Francis Group, LLC